Aluminum flakes are the most important group of effect pigments. The unique coloristical and technical profile—high gloss, excellent hiding and neutral silver shade—allow color designers the use in all shade areas.
Conventional metal effect pigments are produced by treating metal granules with stamping machines. The most common way to produce aluminum flakes is the so called Hall process—a wet milling process which is flattening aluminum granules in a ball milling process. Standard aluminum pigments are produced in “cornflake” and “silverdollar” types depending on the quality and shape of the starting granules and on the milling conditions.
The thickness of aluminum flakes produced by mechanical impact is in general in between 50 nm and 1000 nm. The most common types have a medium thickness of 200-500 nm. So called non-degrading flakes have a flake thickness above 500 nm and thus a higher mechanical stability than thinner flakes. However thick flakes like the non-degrading types have a reduced metallic appearance due to undesired scattering at edges and inferior orientation. Thinner flakes have in general a better coloristical appearance (metallic effect). However due to stronger degradation of the thinner flakes under shear stress the coloristical properties of paints comprising the aluminum flakes are changing for example while pumping the paint in a circulation line.
During the last years extremely thin silver dollars with thicknesses <100 nm have been manufactured. Their sensitivity on mechanical impact is even bigger.
A special type of aluminum flakes is PVD (Physical Vapor Deposition) aluminum, also known as VMP (Vacuum Metallized Pigment), produced by a vacuum process where the aluminum is deposited on a web. After releasing the deposited aluminum from the web, very thin plane-parallel flakes are obtained, with improved mirror-like effects when incorporated into coating systems.
The term “plane-parallel flakes” as used herein relates to flakes having two plane faces which are substantially parallel to each other. Said flakes may be in the form of platelets which may be flat or, especially in case of very thin flakes, bent.
In general, commercially available vacuum metallized pigments have thicknesses from 10-50 nm. Those pigments allow mirror like coatings if they are applied on perfectly flat surfaces. However the extremely thin vacuum metallized pigment (VMP) flakes are very sensitive to mechanical impact. That has been for example described in EP1438360B1, column 2, paragraphs [0008] and [0009]. For that reason, the use of VMPs for example in the automotive industry has been mainly restricted to showcars or expensive special editions.
EP0848735 B1 is describing oxidized aluminum or aluminum-alloy pigments having a content of metallic aluminum of not more than 90% by weight with respect to total weight obtainable by oxidation of platelet-shaped aluminum or aluminum-alloy pigments at a pH from 7 to 12 in a mixture of water, one or more water-miscible solvents, and possibly a base, and heating to the boiling point. The resulting pigments have a core of aluminum which is surrounded by a thick layer of oxidized aluminum. Such pigments lack the metallic silver effect and show yellowish to brownish shades. The thick oxidized aluminum layer is stated to improve the mechanical stability (cf. section [0036-0038]).
WO 02/10290 A1 discloses pigment flakes based on an alumina support layer having a thickness of 50 to 1000 nm, preferably 50 to 150 nm, that is coated with a reflector layer preferably composed of a metallic aluminum layer having a thickness of 10 to 150 nm, preferably 80 to 150 nm. The structure of the flakes provides rigidity and brittle fracture during manufacture and application processes.
Hence, there is a demand for effect pigments, preferably with thicknesses <100 nm, more preferably with thicknesses <50 nm, looking like pure silver with the superior metallic effect of thin silver dollars or vacuum metallized pigments and a mechanical stability and/or shear stability which is increased as compared to the prior art. It has surprisingly been found by the inventors of the present patent application that the mechanical stability and/or shear stability of such effect pigments can be improved by an inner core of oxidized aluminum.
The structure of the thin Al flakes of FIGS. 1 and 2 has been investigated as follows:
A paint film with 0.5% Al-flakes (by weight) has been sprayed on a transparent foil. After hardening of the binder system the flakes were fixed and oriented parallel to the surface. Via use of microtome cuts thin slices of the flakes were generated and first flotated onto a water surface. The thin slices were then investigated via transmission electron microscopy (TEM) and Energy Dispersive X-Ray Spectroscopy (EDXS).